This invention is directed to photosensing systems and, more particularly, to modulated photosensing systems.
Since the advent of solid-state light emitting diodes (LEDs), which are relatively easily modulated by solid-state electronic systems, modulated photoelectric control systems have come into widespread use. Modulated photoelectric control systems, in general, are systems wherein one or more light sensing devices are used to sense the position or nature of one or more objects and, in accordance therewith, control the function of a machine. For example, in a simplified system, a light sensing device may be positioned so as to sense when an object, moved by a conveyor, reaches a particular position, position sensing being accomplished when the object prevents light from a light source impinging on the light sensing device. When this position is reached, the overall photoelectric control system may cause a cutting device to be energized, and the object cut. Alternatively, two light sensing devices can be used to form a smoke detector. In such an embodiment, one light sensing device receives light through a closed system (e.g., light pipe) and the other receives light passing through the atmosphere. A comparison of the outputs of the two light sensing devices provides an indication of the existence of smoke in the atmosphere through which the second light passes.
While the use of modulated photoelectric control systems have become fairly widespread in machine tool and related environments because of their greater useful sensitivity and light immunity characteristics when compared with unmodulated, photoelectric devices, certain disadvantages have made them unuseful in certain environments. For example, the use of remotely located modulated LEDs and their related photosensors has been very limited because of the inability of electronic systems to reliably detect the photosensor's signal. That is, in the past, modulated photosensor outputs connected to a detector via a cable in excess of several feet long have been reduced by cable capacitance to the point where they have become lost in the noise picked up by the cable and, thus, undetectable by conventional amplification and detection circuits. These signals have become undetectable because prior art detectors have, even when balanced, had unacceptably high input impedances.
More specifically, the maximum allowable cable length is inversely proportional to cable capacitance per unit length, the input impedance of the line preamplifier and the operating frequency. Mathematically, these terms can be related by the following formula: ##EQU1## where: L.sub.MAX = the maximum cable length;
F = the modulation frequency; PA1 R = the input impedance of the preamplifier; PA1 C = the cable capacitance per unit length; and PA1 K = a constant of proportionally dependent on the allowable level of signal degradation.
In a typical photoelectric system, F = 15 KHz; R = 10,000 ohms; C = 70pF/foot; and, K = 1.0. For such a system, L.sub.MAX is calculated to be 15 feet. If, however, R can be reduced to 1000 ohms, then L.sub.MAX becomes 150 feet without any additional signal attenuation occurring. While L.sub.MAX can also be increased by lowering the modulation frequency, such lowering results in longer response times and less immunity to standard artificial light. Alternatively, K can be increased, but at the expense of greatly added cost or reduction in system performance. Thus, these latter two solutions are generally considered to be unsatisfactory.
The prior art has attempted to solve the foregoing problem by amplifying the photosensor outputs prior to applying them to the input end of relatively long cables. While this solution is somewhat satisfactory, it has the obvious disadvantage that it increases the size, complexity and cost of the portion of the system related to the production of the photosensor signals. Moreover, amplifiers require power either from a locally located battery or via cables connected to a remote supply; hence the photosensor signal production system becomes an active rather than a passive system, and acquires all of the disadvantages that an active system has over a passive system.
Therefore, it is an object of this invention to provide new and improved modulated photosensing systems.
It is a further object of this invention to provide a modulated photosensing system wherein the output of a photosensor directly connected to one end of a relatively long cable, i.e., one in excess of several feet, is detectable at the other end of the cable.
It is a still further object of this invention to provide a new and improved unbiased modulated photosensing system that includes a low input impedance detector suitable for detecting an unamplified photosensor signal applied to one end of a relatively long cable.
It is a yet another object of this invention to provide a new and improved low input impedance detector suitable for detecting relatively low level signals applied to a relatively long cable.
While modulated photoelectric control systems have a number of advantages over unmodulated photoelectric control systems, in the past, unmodulated photoelectric control systems have retained other advantages. The primary retained advantage has been their ability to directly connect photosensors in series or parallel to perform certain comparing and simple logic-type functions. More specifically, while pulse modulated control systems have proven to have superior sensitivity and light immunity characteristics, in the past, their outputs have not been directly combined (prior to amplification and/or transmission) to perform certain comparing and logic-type functions. For example, their outputs have not been directly combined to provide a comparison smoke detector of the type generally described above.
Therefore, it is yet another object of this invention to provide modulated photosensing systems wherein the outputs of more than one modulated photosensor are directly combined together.
It is a still further object of this invention to provide unbiased modulated photosensing systems suitable for performing certain comparing and logic-type functions by directly combining the outputs of a plurality of modulated photosensors, i.e., combining them without electronically changing them into a binary logic form.
It is a more comprehensive object of this invention to provide modulated photosensing systems adapted to directly perform comparing and logic-type functions and transmit the result over cable lengths in excess of several feet and to provide a low input impedance detector suitable for detecting the low-level logic signals applied to cables of such lengths.